SQL:
外連接和內連接：
左連接或左外連接：包含左邊的表的所有行，如果右邊表中的某行沒有匹配，該行內容為空（NULL）
--outer jion:left join or left outer join
select * from dbo.Project left join dbo.Voice on (dbo.Project.voiceID=dbo.Voice.ID)
右連接或右外連接：包含右邊表的所有行，如果左邊表中的某行沒有匹配，該行內容為空（NULL）
--outer jion:right join or right outer join
select * from dbo.Project right join dbo.Voice on(dbo.Project.voiceID=dbo.Voice.ID)
完全連接或完全外連接：涵蓋左連接與右連接，包含兩張表中的所有行，不匹配的內容為空（NULL）
--outer join:full join or full outer join
select * from dbo.Project full join dbo.Voice on (dbo.Project.voiceID=dbo.Voice.ID)
內連接：
--join or inner join
select * from dbo.Project inner join dbo.Voice on (dbo.Project.voiceID=dbo.Voice.ID)
select * from dbo.Project,dbo.Voice where (dbo.Project.voiceID=dbo.Voice.ID)
select p.ID,p.Name,p.bak,v.NAME from dbo.Project p join dbo.Voice v on(p.voiceID=v.ID)
交叉連接：（1）分為沒有where子句的交叉連接將產生連接所涉及的表的笛卡爾積。第一個表的行數乘以第二個表的行數等於笛卡爾積結果集的大小。
--cross join
select * from dbo.Project cross join dbo.Voice
select * from dbo.Project cross join dbo.Voice where (dbo.Project.voiceID=dbo.Voice.ID)
--兩表關系為一對多，多對一，多對多的連接語句
select p.Name,v.NAME from dbo.Project_Voice as pv left join dbo.Voice as v on v.ID=pv.voiceID left join dbo.Project as p on p.ID=pv.projectID
select p.Name,v.NAME from dbo.Project_Voice as pv left join dbo.Voice as v on v.ID=pv.voiceID left join dbo.Project as p on p.ID=pv.projectID
where p.voiceID=v.ID
LINQ:（概念和結果參考sql語句的）
左連接：
var userInfo = from s in db.Project
join c in db.Voice on s.voiceID equals c.ID into ProjectV
from pv in ProjectV.DefaultIfEmpty()
select new { s.Name,s.bak,pv.NAME,s.ID};
右連接：
var userInfo = from c in db.Voice
join s in db.Project on c.ID equals s.voiceID into PV
from pv in PV.DefaultIfEmpty()
select new { pv.Name,pv.bak,pv.ID,c.NAME};
內連接：
var userInfo = from s in db.Project
join c in db.Voice on s.voiceID equals c.ID
select new { s.Name, s.bak, c.NAME, s.ID };